The present invention generally relates to a wet oxidation process for forming silicon oxide layers and more particularly, relates to a dual-stage wet oxidation process for forming silicon oxide layers wherein different H2/O2 are utilized in two separate stages.
The formation of silicon oxide on a silicon substrate is a frequently conducted process in the fabrication of semiconductor devices. One of the methods for forming silicon oxide is thermal oxidization which is carried out by subjecting a silicon wafer to an oxidizing ambient at elevated temperatures. A common objective of an oxidizing system is to obtain a high quality silicon oxide film of uniform thickness while maintaining a low thermal budget (the product of temperature and time). Methods have been developed to increase the oxidation rate and to reduce the oxidation time and temperature. Two of such methods are the dry oxidation method and the wet oxidation method by using an external torch.
The substances used to grow thermal oxides on a silicon surface are dry oxygen and water vapor. In a dry oxygen reaction, silicon oxide is formed by Si+O2xe2x86x92SiO2, while for water vapor, the reaction is Si+2H2Oxe2x86x92SiO2+2H2. In both cases, silicon is consumed and converted into silicon dioxide.
In a dry oxidation process, silicon dioxide layers can be formed in a temperature range of 400xc2x0 C.xcx9c1150xc2x0 C. The process is typically performed in a resistance-heated furnace or in a rapid thermal processing chamber with heat provided by tungsten halogen lamps. In a typical dry oxidation process, a horizontal furnace tube may be used in which a batch of wafers is introduced into the furnace tube positioned in a slow moving wafer boat and then heated to an oxidation temperature in a ramp-up process. The wafers are held at the elevated temperature for a specific length of time and then brought back to a low temperature in a ramp-down process. In the dry oxidation process, oxygen mixed with an inert carrier gas such as nitrogen is passed over the wafers that are held at an elevated temperature.
A wet oxidation process can be performed by either bubbling oxygen through a high purity water bath maintained at between 85xc2x0 C. and 95xc2x0 C., or by a direct reaction of hydrogen with oxygen producing water vapor in a pyrogenic steam oxidation process.
The thermal budget required to grow a silicon oxide layer to a certain thickness is considerably smaller in a wet oxidation process than that in a dry oxidation process. The wet oxidation process for producing a silicon oxide film can therefore be carried out more efficiently and at a lower cost. However, because of a residual water content, silicon oxide films formed by the wet oxidation process exhibit a lower dielectric strength and has higher porosity to impurity penetration than silicon oxide films formed in a dry oxidation process. As a compromise, wet oxidation process is frequently used in conjunction with dry oxidation process such that a high quality oxide film can be grown with minimized oxidation time required. This is performed by beginning and ending an oxidation process in dry oxygen while using the wet oxidation process for the intermediate stage which reduces the thermal budget while increasing the overall oxide growth rate. By using this dry oxidation-wet oxidation-dry oxidation process sequence, high quality silicon oxide films can be grown on both sides of the oxide layer in order to provide properties of the three-layered film comparable to those of a single layer grown by a dry oxidation process alone.
Another benefit of the wet oxidation process is that the apparatus used for carrying out the wet oxidation may also be used to carry out a dry oxidation process. For instance, as shown in FIG. 1, a wet oxidation apparatus 10 consists of an oxidation chamber 12, an external torch 14, and a conduit 16 that connects the external torch 14 and the oxidation chamber 12 for providing fluid communication therein between. The wet oxidation apparatus 10 further includes conduit 20 for feeding an inert gas into conduit 16 for purging both the conduit 16 and the oxidation chamber 12, conduit 22 for flowing oxygen into the external torch 14 by a carrier inert gas, and conduit 24 for flowing hydrogen into the external torch 14 with an inert carrier gas. An exhaust conduit 28 takes away unused or excess water vapor in the oxidation chamber 12. The flow of gases in conduits 20, 22 and 24 is controlled by valves 30, 32 and 34, respectively.
The convention wet oxidation apparatus 10 shown in FIG. 1 has been used for many years. In a normal silicon oxide growth process, in order to achieve high growth rates while minimizing the thermal budget of the process, the maximum H2/O2 gas mixture ratio of 1.8 is used for producing thick silicon oxide layers, i.e. layers thicker than 2000 xc3x85. At the high H2/O2 gas mixture ratio of 1.8, the partial pressure of water vapor in the reaction chamber is very high which causes a loading effect, i.e., the lesser number of wafers are loaded in the reaction chamber, the poorer is the wafer-to-wafer coating uniformity.
In the conventional thick silicon oxide growth process carried out by the water oxide method, the process is carried out by a single step pyrolysis technique at a high H2/O2 ratio of about 1.8. The gas mixture ratio of 1.8 for H2/O2 is the highest possible within a safety limit without the danger of causing an explosion in the furnace. After the gas mixture is burned in a torch, the high H2/O2 gas mixture ratio produces high water pressure in the furnace tube and thus achieves a high growth rate of silicon oxide. However, the excess water vapor left in the furnace tube does not stop reacting on the plurality of wafers positioned in the furnace until the water vapor is purged out by an inert gas.
The reaction mechanism in the wet oxidation process can be shown as follows:
2H2O+Sixe2x86x92+SiO2+2H2. (The original reaction)
2H2+O2xe2x86x922H2O(H2 reacts with O2 at high temperature in tube)
2H2O+Sixe2x86x92SiO2+2H2 (Secondary reaction occurs before H2O is purged out)
The secondary reaction causes an effect known as the loading effect in which when the furnace tube is loaded only with a few wafer and that the wafers are charged from the top of the boat, the loading effect is very serious in the top than the bottom due to the different gas flow conditions leading to poor wafer-to-wafer uniformity.
The loading effect occurring in a conventional wet oxidation process for growing silicon oxide is shown in FIG. 2. As described above, the excess wet vapor in the furnace tube does not stop reacting even after the main pyrolysis step is completed, until all the water vapor is purged out by an inert gas such as nitrogen. The loading effect becomes more serious in the upper chamber of the furnace than the lower chamber when more wafers are positioned in the upper chamber. As shown in FIG. 2, at the lower end of the total number of wafers processed, a deviation in the thicknesses of the silicon oxide layers formed on wafers positioned in the upper chamber can be as high as 100 xc3x85. This deviation is greatly reduced, as shown by the data in FIG. 2 to about 20 xc3x85, when 144 wafers are loaded in the furnace tube. In a semiconductor fabrication facility, the furnace tube cannot always be loaded full of wafers, it is therefore inevitable that whenever there are only a few wafers loaded in the furnace tube, the uniformity in thickness of the silicon oxide layers formed on the wafers becomes unacceptable.
For instance, data plotted in FIG. 2 are obtained by a conventional wet oxidation recipe in which three burn steps are first carried out; the first burn step for about 2 minutes at an O2 flow rate of 10 liter/min, a second burn step of about 1 minute at a O2 flow rate of 3.5 liter/min, followed by a third burn step for about 1 minute with a H2 flow rate of 3.0 liter/min and an O2 flow rate of 3.5 liter/min. After the three burn steps, the pyrolysis reaction is carried out for a time period of 90xcx9c120 minutes at a H2 flow rate of 8 liter/min and an O2 flow rate of 4 liter/min which represents a H2/O2 gas mixture ratio of 1.6. After the pyrolysis process for growing silicon oxide, an inert gas purge such as nitrogen gas purge is conducted to shut-off the oxygen and hydrogen gas flows.
Data obtained from the conventional wet oxidation recipe shown above indicates a wafer-to-wafer uniformity of large than 1.2% and an upper chamber/lower chamber deviation of about 60 xc3x85. These numbers are not acceptable in an IC fabrication process that has tight tolerances on the device manufactured.
It is therefore an object of the present invention to provide a wet oxidation method for growing silicon oxide that does not have the drawbacks or shortcomings of the conventional methods.
It is another object of the present invention to provide a method of wet oxidation for growing silicon oxide that incorporates simple modifications for achieving an improved process and uniformity.
It is a further object of the present invention to provide a wet oxidation method for growing silicon oxide that does not utilize a single-stage pyrolysis process.
It is another further object of the present invention to provide a method of wet oxidation for growing silicon oxide in a vertical furnace that is carried out by a dual-stage pyrolysis process.
It is still another object of the present invention to provide a wet oxidation process for growing silicon oxide that utilizes a dual-stage deposition process at different H2/O2 gas mixture ratios.
It is yet another object of the present invention to provide a wet oxidation process for growing silicon oxide in a vertical furnace that is capable of producing more uniform silicon oxide layers on wafers positioned in the upper and the lower chamber.
It is still another further object of the present invention to provide a wet oxidation process for growing silicon oxide layers by using a dual-stage pyrolysis process wherein the H2/O2 gas mixture ratio used in the second stage is at least ⅓ smaller than that used in the first stage.
It is yet another further object of the present invention to provide a wet oxidation process for growing silicon oxide in a vertical furnace in which a first stage pyrolysis process utilizes a H2/O2 gas mixture ratio larger than 1.5 while a second stage pyrolysis process utilizes a H2/O2 ratio smaller than 1.2
In accordance with the present invention, a method for forming silicon oxide by wet oxidation that utilizes a dual-stage pyrolysis technique is disclosed.
In a preferred embodiment, a method for forming silicon oxide by wet oxidation can be carried out by the operating steps of first providing an oxidation chamber equipped with a torch and a conduit providing fluid communication between the chamber and the torch, positioning a plurality of silicon wafers in the oxidation chambers flowing a first H2/O2 mixture that has a first H2/O2 ratio into the torch and feeding water vapor generated through the conduit into the oxidation chamber for a first time period, flowing a second H2/O2 that has a second H2/O2 ratio into the torch and feeding water vapor generated through the second conduit into the oxidation chamber for a second time period. The second H2/O2 ratio is smaller than the first H2/O2 ratio by at least ⅓ of the value of the first H2/O2 ratio.
In the method for forming silicon oxide for wet oxidation, the second time period is smaller than the first time period. The second time period is between about 10% and about 40% of the first time period. The first time period is between about 50 min. and about 90 min., and the second time period is between about 10 min. and about 30 min. The first H2/O2 ratio may be larger than 1.2 and the second H2/O2 ratio may be smaller than 1.2. The first H2/O2 ratio may also be larger than 1.6 and the second H2/O2 ratio may also be smaller than 1.1. The method may further include a nitrogen purging and oxygen annealing process. The method may further include a nitrogen purging process for stopping the flow of H2/O2.
In another preferred embodiment, the present invention discloses a method for growing silicon oxide layers on silicon wafers by a wet oxidation process which includes the steps of providing a wet oxidation chamber equipped with a torch and conduit providing fluid communication between the chamber and the torch, positioning a plurality of silicone wafers in the wet oxidation chamber, flowing a first H2/O2 mixture that has a H2/O2 ratio large than 1.5 into the torch and fed water vapor generated through the conduit into the wet oxidation chamber for a first time period, and flowing H2/O2 mixture that has a H2/O2 ratio smaller than 1.2 into the torch and feed water vapor generated through the conduit into the wet oxidation chamber for a second time period.
The method for growing silicon oxide layers on silicon wafers by a wet oxidation process may further include the step of positioning each of the plurality of silicon wafers horizontally in a vertical stack. The method may further include the step of flowing H2 and O2 individually into the torch for mixing in the torch and reacting paralytically to produce water vapor. The first time period may be at least 2 times of the second time period, or the first time period may be at least 3 times of the second time period. The first time period may be more than 40 min. and the second time period may be less than 20 min. The second H2/O2 ratio may be smaller than the first H2/O2 ratio by at least ⅓ of the value of the first H2/O2 ratio. The method may further include the step of depositing silicon oxide layers on the plurality of silicon wafers to a thickness of at least 1000 xc3x85, or to a thickness of a least 2000 xc3x85.
In still another preferred embodiment, a dual-stage wet oxidation process for forming silicon oxides layers can be carried out by the operating steps of first providing an oxidation chamber equipped with a torch and a conduit and providing fluid communication between the chamber and the torch, positioning a plurality of silicon wafers horizontally in a vertical stack in the chamber, flowing a H2/O2 mixture that has a H2/O2 ratio large than 1.5 into the torch and feeding water vapor generated through the conduit into the oxidation chamber for more than 40 min. in a first stage for forming a first thickness of silicon oxide, and flowing a H2/O2 mixture that has a H2/O2 ratio smaller than 1.2 into the torch and feeding water vapor generated through the conduit into the oxidation chamber for less than 20 min. in a second stage for forming a second thickness of silicon oxide.
The dual-stage wet oxidation process for forming silicon oxide layers may further include the step of forming the first and the second thickness of silicon oxide on the plurality of silicon wafers to a total thickness of at least 2000 xc3x85. The method may further include a nitrogen purging and an oxygen annealing step after the first and the second stage of forming silicon oxide.